Echelon LonPoint Application and Plug-In - Manual (Page 34)

3-8 Digital Input/Counter Functional Block
Frequency
Measures the frequency of the input signal in the range of 1Hz to 20kHz.
Minimum resolution of the frequency measurement is <0.5% over the full
measurement range. The Analog network variable reports the instantaneous
measured frequency or a translated representation of it. The Digital network
variable is not used with this processing option.
On startup the Analog output network variable is not immediately available as
the input is auto-ranged. This process takes up to four seconds.
The frequency measurement may be modified using a translation table. The
translation table operates in the same manner as the translation table in the
Analog input functional block found in the AI-10 LonPoint Module. The
translation table is in turn followed by a filtering function. The filter averages
over N samples, where N is the configured filter length. The supported range of
N is 1 to 255. The equivalent filter time constant for this range is approximately
0.5 seconds to 50 seconds. No filtering is done if N equals 1.
If no translation is applied, an appropriate network variable type for the Analog
output network variable in this mode is SNVT_freq_f. If translation is applied
then the appropriate floating point SNVT for the process variable being
measured should be used (e.g. SNVT_flow_f).
The input cannot be configured for dry contact when the frequency processing
option is selected. Debounce is not supported. The hardware input may be
configured for voltage input (5V, 12V, 24V, or 31V only).
Using The Control Network Variable
The Control network variable can be driven from another device on the network
or an HMI application. It is sequenced in a way that allows count values to be
sampled while continuing to count without interruption. It applies to the Count,
Repeating Count, and On Time processing options. The Control network
variable type is UNVT_count_control.
The following summarizes the input values to the Control network variable:
RUN Enables event counting. The count value is output to
the Analog network variable. The send-on-delta value
is used to throttle the network variable, so the
network variable is not necessarily propagated with
every count.
CLEAR Copies the internal count to the Analog network
variable and then clears the internal count. Events
continue to be counted, but the Analog network
variable is not updated. This allows an HMI
application to read the Analog network variable
while the Control network variable is set to CLEAR.
The HMI application can set the Control network
variable to RUN after reading the Analog network
variable.